Ordinary Chondrites


Chondrites are divided into 3 classes, ordinary, carbonaceous, and enstatite, which are further divided into groups, 12 in all. These groups are defined by properties that include bulk compositions, isotopic compositions, oxidation states, and textures. Ordinary chondrites (OCs) are divided into three groups: H group has high total Fe contents (FeO as Fa and metallic Fe), L group has low total Fe, and LL group has lowest metallic Fe as well as low total Fe.

Early in their histories meteorite parent bodies were heated by radioactive decay and overburden pressures, among other sources, and their component rocks were thermally modified (metamorphism), some to the point of melting. Heating of chondritic parent bodies produced levels of metamorphism that range from complete recrystallization to those that show little change. A summary of the diagnostic mineralogic and chemical characteristics of ordinary chondrites is given in the table below.

HLLL
ExamplesDhajalaKhokarSemarkona, NWA 1756
Chemistry1Abundance patternsRLE = 0.8 x CI; Fe2+/Fetotal~0.38RLE = 0.8 x CI; Fe and siderophiles ~0.75 x CI; Fe2+/Fetotal~0.66RLE = 0.8 x CI; Fe and siderophiles ~0.6 x CI; Fe2+/Fetotal~0.88
Mean Mg/Si0.960.930.93
Mean Al/Si0.0690.0680.065
Mean Ca/Si0.0520.0500.048
Mean δ18O4.14.64.9
Mean δ17O2.93.53.9
Approximate chondrule abundance (vol%)60-80 (matrix is dominately olivine)
ChondruleSize (mm)0.30.70.9
Types2PO (23), POP (48), PP (10), granular (3), BO (4), RP (7), CC (5)
Ni-Fe metal (vol%)15-194-90.3-3
OlivineFa (mol. %)16-2022-2627-32
FeO/MnO30-4046-5550-63
Pyroxene composition
Fs (mol. %)
14.5-18.519-2223-26
Chromite Cr/(Cr+Al) (mol.)0.91 (H3) → 0.86 (H6) 0.93 (L3) → 0.88 (L6) 0.90 (LL3) → 0.87 (LL6)

NOTES. 1 RLE = refractory lithophile elements, CI = CI chondrites. 2 Chondrule types (percentage): PO = porphyritic olivine; POP = porphyritic olivine-pyroxene; PP = porphyritic pyroxene; BO = barred olivine, RP = radial pyroxene, CC = cryptocrystalline.

Van Schmus and Wood (1967) devised a clever classification scheme that combined metamorphic changes with bulk compositions and assigned the petrologic types 1 through 6. Only types 3 through 6 are used for ordinary chondrites, 1 and 2 are theoretical and have not been found. The most primitive of these is type 3 and the remaining types reflect increasing metamorphic grade through type 6. The original classification system did not include a type 7, although researchers have used a 7 designation to account for chondrites that have no relict chondrules. In other words, metamorphism progressed to the point that all chondrules were completely recrystallized. The table below summarizes some of the critical changes that are more easily recognized by observations and analyses.

CriterionPetrologic Type
3456
Olivine homogeneity> 5% mean deviations<5%homogeneous
Low-Ca pyroxene
predominantly monoclinic>20% monoclinic≤20% monoclinicorthorhombic
Feldsparminor primarysecondary <2 μm grainssecondary 2-50 μm grainssecondary >50 μm
Chondrule glassclear, isotropicdevitrifiedabsent
Matrixopaque to transparenttransparent, recrystallized (coarsening in 4 to 6)
Chondrule-matrix
definition
sharply definedmoderately diffusediffuse
Carbon (wt.%)0.2-1<0.2
Water (wt.%)0.3-3<1.5

Type 3 to 4 chondrites possess characteristics that have the potential for finer tuning of metamorphic effects. Because of the subtle differences in textures and compositions in these inhomogeneous meteorites, classification into subtypes (3.1-3.9) between types 3 and 4 can be made that are useful in estimating heating/cooling rates, temperatures reached, and elemental diffusion rates for materials on or near the surface of the respective parent bodies. For example, FeO contents (Fa molecule, the FeO-rich end member of the dichotomous FeO-MgO series in olivines) in unequilibrated olivines are very heterogeneous and may vary between Fa1 to Fa60 in the least equilibrated subtypes. Fa contents in subtypes < 3.5 show a deviation from the mean of ± Fa15, subtype 3.9 has a deviation of ± Fa3 from the mean. Type 4 olivines have a deviation of <Fa1 or that within the precision microprobe analyses.

The methods used for determining these subtypes in unequilibrated chondrites are rather labor intensive and time consuming. Recently, Grossman and Brearley (2005) refined an analytical technique that is very reliable and uses less instrumental and observational time (figure below from their paper). This technique is based on chromium (Cr) content and distribution in FeO-rich olivine, which is very sensitive to metamorphic conditions and changes dramatically between subtypes 3.0 and 3.2. For example, Semarkona, the only 3.0 ordinary chondrite and the least equilibrated known, has a mean Cr2O3 content in ferroan olivines of 0.50 wt % and a range of 0.4 to 0.5 wt %. The LL3.2 Bishunpur chondrite has a Cr2O3 content of 0.22 ± 0.16 wt % in its ferroan olivines; the more equilibrated LL3.4 Chainpur has 0.06 ± 0.06 wt % Cr2O3. Those > 3.6 are devoid of measurable Cr2O3 (< 0.02 wt % or 200 ppm via the electron microprobe), chromium was entirely driven out of the olivine structure and into the surrounding matrix in chondrules of subtype >3.6 by metamorphic heating.

Example Cr2O3 analyses for low petrologic types of ordinary chondrites (Grossman & Brearley, 2005).

Determination of low chondrite types based upon abundance and standard deviation of Cr2O3 analyses (Grossman & Brearley, 2005).

The Grossman and Brearley (2005) subdivisions can be extended to by characterizing the range in olivine Fa content and the mean olivine Cr2O3 content. Bunch et al (2012) recommend the following guidelines for assigning subtypes to unequilbrated LL and L chondrites. Use the Grossman and Brearley (2005) scheme for subtypes 3.0 to 3.2. For subtypes 3.3 to 3.6, which have a wide range in Fa contents, use the Cr2O3 contents in ferroan olivine as follows:

Variation in Cr2O3 contents in olivine of L chrondrites and the designation of subtypes 3.1 through 3.5; mean values are indicated by the central tick mark.

Variation in Cr2O3 contents in olivine of LL chrondrites and the designation of subtypes 3.1 through 3.5; mean values are indicated by the central tick mark.

Subtypes 3.7-3.9 have undetectable Cr in ferroan olivine (but still measurable Cr in the most magnesian olivines within chondrules). Subdivision of these is accomplished by utilizing the range in Fa contents for a representative sampling of all olivines in a specimen as follows:

Variation in Fa and mean Cr2O3 contents in olivine of L chrondrites and the designation of subtypes; mean Fa values are indicated by the central tick mark.

Variation in Fa and mean Cr2O3 contents in olivine of LL chrondrites and the designation of subtypes; mean Fa values are indicated by the central tick mark.

This scheme is necessarily subjective to some degree, but we believe that it can be used to assign decimal subtypes within ±0.1. Depending upon factors such as intrinsic heterogeneity in the meteorite (including the possibility of clasts of different subtypes mixed together, unequilibrated crystal cores, etc.), and also attention to quantitative Fa and Cr analyses, different classifiers may arrive at slightly different subtype estimates for the same specimen.

Compositional zoning in olivine and pyroxenes of unequilibrated chondrites is readily observed in scanning electron microscope (SEM) and electron microprobe (EMP) backscattered electron images. These images are created when high-energy electrons strike the sample surface and are backscattered according to the sample's atomic number or density. Bright areas in the images indicate dense (Fe-rich) material and dark areas indicate areas of less dense or MgO-rich material.